Ll. Hyde et H. Underwood, Effects of melatonin administration on the circadian activity rhythm of the lizard Anolis carolinensis, PHYSL BEHAV, 71(1-2), 2000, pp. 183-192
The green anole, Anolis carolinensis, is the most thoroughly studied North
American lizard species, yet little is known about the circadian system of
this species. Studies in several lizard species, including Anolis, support
the hypothesis that the pineal organ is the site of a major circadian pacem
aker that controls the phasing and frequency of circadian clocks located el
sewhere. We examined the effects of exogenous administration of the pineal
hormone, melatonin, to determine if the pineal secretion of melatonin could
be the mechanism whereby the pineal communicates with the rest of the circ
adian system in Anolis. Continuous melatonin administration via subcutaneou
s silastic implants to pineal-intact anoles exposed to light-dark (LD) cycl
es 6:18 and LD 6:20 and to pinealectomized anoles entrained to LD 6:18 comp
letely suppressed activity that preceded lights-on suggesting a direct inhi
bitory (masking) effect on activity. Continuous exogenous administration of
lower dosages of melatonin to anoles expressing their endogenous circadian
activity rhythm (free-running) in dim continuous light caused either arrhy
thmicity, a shortening of the free-running period, bouts of arrhythmicity a
nd rhythmicity, or a suppression of activity. It was concluded that melaton
in can affect both the expression of the activity rhythm as well as the cir
cadian oscillator driving the activity rhythm. The results support the hypo
thesis that the circadian pacemaker in the pineal organ controls other (sub
ordinate) circadian clocks via the rhythmic synthesis and secretion of mela
tonin. (C) 2000 Elsevier Science Inc. All rights reserved.